This invention relates to a system for stopping a spindle at a predetermined rotational position in which a specified point on the spindle is stopped at the predetermined position, and more particularly to a control system of the type described in which a position sensor mounting error is corrected electrically to enable the spindle to be stopped with a high degree of accuracy.
Some machine tools which are known in the art have an automatic tool changing function which allows machining to be performed while a variety of tools mounted on the machine are interchanged automatically. The tool changing operation proceeds as follows. First, a magazine holding a number of tools is revolved to bring a vacant tool holding portion of the magazine into position directly above a spindle mechanism. The spindlle mechanism, which is grasping an old tool to be exchanged for a new one, is then moved forwardly, after which the magazine positioned above the spindle mechanism is lowered to permit the old tool to be received and grasped by the vacant tool holding portion of the magazine. The spindle mechanism is then retracted so that the old tool separates from the spindle, thereby transferring the old tool to the magazine. Next, the magazine is revolved to bring a desired new tool into position in front of the spindle, and the spindle mechanism is moved forward to allow the spindle to receive and to grasp the new tool. Finally, the magazine is raised away from the spindle to complete the tool change operation.
It is necessary in the tool change mechanism of the foregoing type that a prescribed part of the spindle, such as a keyway, be stopped accurately at the correct rotational position in order to permit the fitting portions of the spindle and tool to mate with each other smoothly. More specifically, a keyway is formed in the spindle and a key is formed on the tool in order to mate with the keyway. The smooth mating of the spindle and tool requires that the spindle be positioned and stopped in order to provide the correct alignment of key and keyway. Meeting the above requirement necessitates a high spindle positioning accuracy of .+-.0.05.degree. with respect to the angle of rotational of the spindle.
Conventional automatic tool change mechanisms have a photoelectric detector or with limit switch mechanism which detect the rotational position of the spindle keyway in order to facilitate the smooth mating of the spindle and tool. The arrangement is such that the spindle is brougt to a stop at the prescribed position by the application of a mechanical brake which is actuated in response to a signal from the position detection means.
The foregoing apparatus employs a stopping mechanism that experiences wear with a long period of use since the mechanism relies upon a mechanical pin or brake or the like. Such wear, particularly of a brake shoe or pin, makes it progressively more difficult to stop the spindle at the predetermined position, and the result is that the automatic changing of tools cannot proceed smoothly.
Accordingly, there is a need for a control system which is capable of stopping a spindle at a predetermined rotational position with a high degree of accuracy purely by electrical means, without relying upon a mechanical pin or mechanical brake mechanism to stop the spindle when performing an automatic tool change operation.
To this end, the assignee of the present invention have already proposed a control system, disclosed in copending U.S. patent applications by Kohzai et al. with Ser. No. 190,659, entitled "Spindle Rotationl Control System", and by Fujioka with Ser. No. 215,631, entitled "Control System for Stopping Spindle at Predetermined Rotational Position", both assigned to the assignee of the present invention, for stopping a spindle in such a manner that a specified point on the spindle is brought to a stop at a predetermined rotation position purely through electrical means. The proposed control system includes a position sensor and an orientation control circuit. The position sensor generates a signal having a prescribed level, such as zero volts, when the specified point on the spindle arrives at the predetermined rotational position, and generates a positive or negative voltage when the specified point is in the vicinity of the predetermined rotational position, the polarity of the non-zero voltage depending upon whether the specified point is to the right or left of the predetermined position. The orientation control circuit stops the spindle drive motor when the output signal from the position sensor attains the value of zero volts, thereby stopping the specified point on the spindle at the predetermined rotational position.
In the proposed system the position sensor is mounted within the spindle mechanism and is emersed in machine oil which fills the interior of the spindle box. It is therefore extremely troublesome to positionally adjust or remove the position sensor once it has been mounted. Although the sensor is carefully mounted after precise mechanical positioning within the degree of accuracy of .+-.0.05.degree., inspection after assembly occasionally reveals that the sensor has been shifted from its correct position. In such cases the error must be corrected by finely adjusting the position at which the sensor is mounted.